1. Field of the Invention
The present invention relates to an apparatus for polishing wafers such as are made of single crystals of silicon or a compound semiconductor, or are made of ceramic like quartz and more particularly, to the apparatus for polishing wafers wherein the wafers are polished in batch processing mode while being adhered to a polishing plate.
2. Description of the Prior Art
There is well known an polishing apparatus, for example, as illustrated in FIG. 5 as polishing apparatuses of the above-mentioned kind. The apparatus, which is of a fluid pressure type as for a polishing down load, comprises a hollow top ring 22 fast held to the lower end of the rotary shaft 21, a flexible thin plate 23 secured to the lower end of the top ring 22 so as to form a sealed space 24 and a supply pass 25 for pressurized fluid arranged inside the rotary shaft 21, which communicates with the sealed space 24.
The above-mentioned thin plate 23 is a pressure diaphragm made from a soft material such as rubber and like that and for use in pressurizing the polishing plate 26 made from a plate with high rigidity such as a grass plate or ceramic plate, which the wafers W are mounted to.
The operation of the polishing apparatus will now explained below with reference to FIG. 5. The polishing plate 26, to which the wafers W are mounted, is placed in position on the polishing pad 32 mounted on the platen 31, then the top ring 22 is moved down to a position at which the surfaces of the wafers W to be polished are very close to and in a parallel relation with the polishing pad 32, and further in succession thereto pressurized fluid is supplied to the sealed space 24 from a supply source (not shown) of pressurized fluid.
In this condition, a load with uniform distribution is applied across the thin plate 23 by fluid pressure of the sealed space 24 and thereby both the thin plate 23 and the polishing plate 26 are displaced toward the side of the polishing pad 32, so that the surfaces of the wafers W to be polished are pressed to the polishing pad 32 for polishing.
There is a problem inherited by the polishing apparatus, which is of a pressure type of a load with uniform distribution across the thin plate 23 and the polishing plate 26, which is that local sinks by deformation occur in areas in the lateral expanses of the plates 23, 26 where the wafers W are not mounted, as illustrated in FIG. 6. The local sinking causes deterioration of the flatness of the wafers W and in more particular, opposite portions along the periphery of the wafers in the direction of rotation of the polishing plate 26, that is, the hatched portions of the periphery of the wafers are more polished off than the other portions thereof as shown in FIG. 7 only to affect the flatness unfavorably, so that it is very hard to acquire polished wafers with high flatness.
There has been used as the above-mentioned polishing plate 26 the same made from a high-rigidity material having such a thickness that the local sinking does not occur by deformation during wafer polishing operation.
A harmful influence to the work efficiency was observed clearly, however, with such a polishing plate 26, which is heavy to a considerable extent, in handling, especially, positioning on the polishing pad 32 or removing from the polishing pad 32.